An electrically conductive paste is used to form wiring on an electronic component such as a chip resistor, a chip capacitor, a solar cell; and a mounted electronic component such as a printed-circuit board and a substrate having a through hole. Further, it may be used for an electrode and wiring connected to a transistor which controls pixel switching in a display. Many of the current electrically conductive pastes, in which a silver paste having good oxidation resistance is used, have the following problems: silver is expensive; and, in addition, a migration defect may easily be formed in fine pitch wiring. Therefore, attempts have been made for obtaining a low resistance and reliable wiring structure by printing and baking copper paste in which copper is substituted for silver.
For example, Patent Literature 1 discloses an electrically conductive copper paste comprising a thermosetting resin, an organic carboxylic acid compound and a chelating agent as essential ingredients, and having a mean particle diameter of copper particles of 7.0 μm to 9.0 μm.
Patent Literature 2 discloses an electrically conductive copper paste comprising copper particles, a metal salt, polyols, an epoxy resin and imidazole as essential ingredients.
Patent Literature 3 discloses an electrically conductive copper paste having a mean particle diameter of copper particles of 0.1 μm to 1.0 μm, in which a SiO2 gel coating with a thickness of 100 nm or less is applied on the surface of a copper particle.
Patent Literature 4 discloses an electrically conductive copper paste having a particle diameter of copper particles of less than 0.5 μm, in which the surface of a particle is protected from oxidation with dibutylhydroxytoluene.
Patent Literature 5 discloses an electrically conductive copper paste in which 0.1 atomic percent to 10 atomic percent of silicon is contained inside copper particles.
Patent Literature 6 discloses an electrically conductive copper paste in which copper particles contain aluminum.
Patent Literature 7 discloses an electrically conductive copper paste in which copper particles contain bismuth and magnesium.
Patent Literature 8 discloses an electrically conductive copper paste comprising copper flakes having a mean flake diameter of 0.1 nm to 0.3 nm and silver particles having a mean particle diameter of 1 nm to 100 nm.
Patent Literature 9 discloses a copper powder in which a copper oxide is formed on the surface of a copper particle.
Patent Literature 10 discloses an electrically conductive copper paste comprising copper particles containing phosphorus or copper particles having a surface coated with silver, and comprising flux, glass particles.
Patent Literature 11 discloses an electrically conductive copper paste comprising a thermosetting resin, salicyl alcohol, lead, a lead compound, bismuth and a bismuth compound in addition to copper particles.
However, the above conventional electrically conductive copper pastes were composed of metal particles having a flat flake-like shape and those having a sharp needle-like shape. Because of this, a clog may occur when performing screen printing, which leads to the decrease in metal particle density in wiring and may cause spreading of a wiring pattern after baking. Further, an increased void volume may cause poor sintering property, resulting in the increase in wiring resistance after baking.
Moreover, when performing air baking of conventional silver pastes, binder resin reacts with air, and the amount of residual resin in wiring after baking can be reduced as much as possible. Thereby the increase in wiring resistance due to a residual resin can be minimized. In contrast, an electrically conductive copper paste needs to be baked in an inert gas or in vacuum because copper is susceptible to oxidation under an oxygen-containing atmosphere. In this case, a problem may arise, i.e., a binder resin component may remain in wiring due to insufficient oxygen, resulting in poor sintering property and increased wiring resistance.
Even if an electrically conductive copper paste is baked in an inert gas or in vacuum, oxidation of copper particles is difficult to avoid, resulting in increased wiring resistance.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2008-130301
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2008-226727
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2009-79269
Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2009-146890
Patent Document 5: Japanese Unexamined Patent Application, Publication No. 2010-13726
Patent Document 6: Japanese Unexamined Patent Application, Publication No. 2010-161331
Patent Document 7: Japanese Unexamined Patent Application, Publication No. 2011-26631
Patent Document 8: Japanese Unexamined Patent Application, Publication No. 2011-44509
Patent Document 9: Japanese Unexamined Patent Application, Publication No. 2011-94236
Patent Document 10: Japanese Unexamined Patent Application, Publication No. 2011-171274
Patent Document 11: Japanese Patent No. 4396134